Why Girivalam dates span two days

A lunar day, not a calendar day

The Hindu calendar counts days by the moon as well as the sun. A tithi is the time the moon takes to move another 12° ahead of the sun in the sky. There are thirty tithis from one new moon to the next. The fifteenth is Pournami, the full moon: the period when the moon is between 168° and 180° ahead of the sun.

The moon’s speed varies around its orbit, so a tithi lasts anywhere from about 19 to 26 hours, and it can start at any moment of the day or night. A calendar day, by contrast, always runs midnight to midnight. So a tithi usually starts on one date and ends on the next.

An example

Our Pournami calendar calculates the October 2026 full moon (Aippasi Pournami) as:

• Begins: Sunday 25 October 2026, about 11:56 AM IST
• Ends: Monday 26 October 2026, about 9:41 AM IST

Calendars write this as “25–26 October”. Any time in that window counts as Pournami. Most pilgrims walk on the night of the 25th, inside the window.

Which day should I walk?

• Start after the tithi begins and finish before it ends. That is the traditional rule of thumb.
• Prefer the night. It is cooler, and it is when most pilgrims walk. If the tithi runs from morning to morning, the night in between is ideal.
• If the tithi starts late at night, many pilgrims walk the next evening instead, while it is still in effect.
• Leave a margin. On crowded full moons the walk takes longer; check the end time.

Why printed calendars sometimes differ

Printed panchangams differ slightly in their astronomical models and reference points, so they can disagree by a few minutes. Rounding and daylight conventions can occasionally push a date one way or the other. Our times are calculated from the positions of the sun and moon for Indian Standard Time. For 2026 our start dates match the published Girivalam calendar at thiruvannamalai.in. The method is on our Sources page.

The same idea, for festivals

Festivals tied to a nakshatra (lunar mansion), such as Karthigai Deepam on the Krittika nakshatra, have the same problem. Sometimes calendars disagree on which day a festival falls. That is happening for Karthigai Deepam 2027, which is listed as either 11 or 12 December. We show both until the temple decides. See Karthigai Deepam 2027.